(Castanea dentata) in Ontario - Ministry of Natural Resources ...

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Recovery Strategy for the American Chestnut in Ontario level (Milgroom and Cortesi 2004). Medium or large-scale experiments have been completed in West Virginia, Connecticut, Virginia and Wisconsin where up to hundreds of trees and thousands of cankers were inoculated with hypovirulent isolates, with little evidence of effective biological control of blight. Various characteristics of the fungal viruses, the pathogen, and the trees are thought to determine the success or failure of hypovirulence (Milgroom and Cortesi 2004). Knowledge of these factors, such as tree, site, and climate characteristics, and their influence on the epidemiological aspects of blight are often poorly understood (Griffin 1989, Griffin et al. 1991, Brewer 1995). Hypovirulent isolates have been found in various locations in North America, but the only region where hypovirulence has been effective is in Michigan where it occurs naturally and in some places trees are remarkably healthy (Milgroom and Cortesi 2004). Populations of chestnut and blight in Michigan are similar to those in Ontario, where one hypovirulence-associated virus, CHV-3, has been associated with healing cankers and infected isolates of the pathogen (Melzer and Boland 1999). However, the role of hypovirulence in Ontario is less clear than in Michigan. Other strategies for the biological control of chestnut blight have also been evaluated. The use of mud packs directly on cankers is thought to be effective because of the activity of micro-organisms in the soil that affect growth and development of the pathogen in the canker. These micro-organisms may offer an opportunity for alternative approaches to biological control. There is considerable interest in the potential for identifying or breeding American Chestnut that is resistant to chestnut blight. Evolutionary theory suggests that some resistant trees may be present in an otherwise susceptible population of a species, and that these resistant trees may survive in remnant populations following pandemic diseases. Naturally-occurring resistant trees would be an important discovery for the recovery of this species, and differences in susceptibility have been observed among some individuals (Griffin 2000). Unfortunately, it can be difficult to distinguish between resistant trees and trees that have simply escaped disease and there have been no confirmed examples of American Chestnut that are resistant to chestnut blight. The relatively high proportion of trees in southern Ontario that do not have symptoms of chestnut blight is encouraging. There is also considerable interest in breeding resistant American Chestnut trees through interspecific hybridization with Chinese and Japanese Chestnuts, followed by recurrent back-crossing to the American species and selection of resistant individuals. It is anticipated that this procedure will result in progeny that are highly resistant to chestnut blight and are at least 94 percent American Chestnut in other characteristics. While this is expected to produce blight resistant trees for planting, there is no information on the durability of resistance as trees mature. The American Chestnut Foundation has a large and established program in breeding for disease resistance in chestnut and outplanted seeds from their breeding program to test for blight resistance in three national forests in 2008. Breeding programs are also established at several universities and government research stations in the United States. The Canadian 42
Recovery Strategy for the American Chestnut in Ontario Chestnut Council has initiated a disease resistance breeding program in southern Ontario that hopes to build on the efforts of the American Chestnut Foundation and other institutions and to incorporate germplasm that is adapted to this region with disease resistant American Chestnut breeding material. In addition to this traditional approach to breeding resistant chestnut, scientists in the United States are evaluating the potential for genetic engineering of American Chestnut with disease resistance genes from other organisms. 43
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(Castanea dentata) in Ontario - Ministry of Natural Resources ...

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